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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Output transformers a cheap alternative?

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#142
kodabmx said:
Did you actually physically try it? Or are you basing that opinion on something unrelated to physically trying it?
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No, I did not, my budget is very limited, and I am poor enough to try every idea posted on the forum, plausible or not.

My opinion is based on my calculations and measurements provided by others, included you.

kodabmx said:
My opinion is shared by others here, the results are excellent. The oscilloscope traces are cleaner than the Hammond 1650N I was using as a baseline. I posted traces earlier in this thread. including a clean sinewave at 125kHz.
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I have not a clue about that transformer, but others are very disappointing, see for yourself SAC Thailand

Why Hashimoto or Tamura transformers "must" be better than a properly designed and wounded DIY one? 😎 😀
 
#147
kodabmx said:
Yes, but those are SE, and (I think) we're all saying they only work for PP or parafeed, eliminating DC core saturation 🙂
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I know those are SE transformers, but was the only measurements I could found on the web.

You have provided another reason against toroidal PTs. 😛

TonyTecson said:
thanks peter...i wish i had access to those...

core saturation is very real at the low frequencies, that is why SE traffos tend to be bigger and heavier than those for pp...pp opt's can also saturate at low frequencies...
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Yes, you are right, both SE and PP can saturate at low frequencies, but SE tend to be bigger and heavier because they must allow two magnetic fields, one DC and other AC.
 
#149
You're pretty stubborn kodabmx 🙄
Toroidal PT's don't have "better HF" as confirmed by my measurements yesterday.
With really low Rp tubes they have merely "acceptable" HF but not more than that, caused by excessive leakage inductance.
Even a well wound EI output transformer will have better HF response.
Do you think that a company like Plitron wind their output transformers like power transformers? 😡
 
#150
If you say 50kHz sine from a 12k:8r (120+120:6+6) driven by 6P1P with no NFB is "merely acceptable" I'd love to see your set up! And, for the record, I've used Hammond 1650N and Triad VPT18-13800 (secondaries in parallel) on the same KT88 amp, and the HF response was superior from the VPT. Also, I couldn't tame the ringing on a 10kHz square with the Hammond, but the Triad hardly needed any correction.

I'm not trying to be stubborn, I'm merely pointing out that the results and cost savings equate to more than "merely acceptable" IMHO. If you're not an "audiophool", and you're on a tight budget, these ARE a viable option for a low power design.
 
#152
toroid traffos used in a tube output is less forgiving of dc current imbalance,
and can easily saturate because unlike EI's with 3 air gaps, toroids do not have them..
so keep this in mind always...

a push pull plate chokes, EI's with the toroid OPT's cap coupled to the chokes can be done, but then this will no longer be low cost...
 
#154
kodabmx said:
If you say 50kHz sine from a 12k:8r (120+120:6+6) driven by 6P1P with no NFB is "merely acceptable" I'd love to see your set up! And, for the record, I've used Hammond 1650N and Triad VPT18-13800 (secondaries in parallel) on the same KT88 amp, and the HF response was superior from the VPT. Also, I couldn't tame the ringing on a 10kHz square with the Hammond, but the Triad hardly needed any correction.

I'm not trying to be stubborn, I'm merely pointing out that the results and cost savings equate to more than "merely acceptable" IMHO. If you're not an "audiophool", and you're on a tight budget, these ARE a viable option for a low power design.
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I guess you was right about the standard toroids. I measured a small 31:1 toroid myself and the performance is very nice, even better then i see usual with EI or C-cores from so called "dedicated" winders. Measurements i did with a source of 2x 1150 Ohm and secondair 8 Ohm, response -2dB 2Hz-100kHz
Even the copper loss is low, less then 0,2dB
upwvwkcqhox6.png
 
#156
melkert said:
I guess you was right about the standard toroids. I measured a small 31:1 toroid myself and the performance is very nice, even better then i see usual with EI or C-cores from so called "dedicated" winders. Measurements i did with a source of 2x 1150 Ohm and secondair 8 Ohm, response -2dB 2Hz-100kHz
Even the copper loss is low, less then 0,2dB
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They look too good to be true 😱

Can you share measuring circuit, power level, what you drove them with, what was your load?

Any DC imbalance (even a few mA) or it was just "kind to transformers" pure AC?

I suspect a couple Real World tubes will always be at least slightly mismatched and Toroids do not take kindly to that.

Hey, even *Power* toroids , fed Mains voltage, twist and scream (ok, "buzz" ) with some "DC component" (just waveform assymetry) is mixed with Mains AC, don´t see how Output ones, even worse mains ones "used as" outputs can avoid that fate.

Meaning I think your test setup was "too gentle".
 
#158
Running even Garter bias is worth a shot, my experience is very consistently good, the EL86 amplifier I put together is easily within .5mA, and in the past with the 6AS7 it was just as good. You have to match your resistors but that's pretty trivial. Nice and easy, same thing as individual cathode resistors, just doubled up. Only real drawback is on high bias tubes they waste a bit of voltage, but on stuff like the EL86, EL84, or even low-and-hot schemes they can be made to work. Even on the 6AS7 at a 100mA, 100V, -32 grid it isn't too bad, since you're already wasting some efficiency by going triode anyway.
 
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#159
JMFahey said:
So this thread is about saving $20 on transformers by spending $200 extra on tubes 😉

What am I missing? 😉
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What you are missing is half of what has been said. No tube matching is required if you use servo balancing of some sort, and the type of tubes suitable are to be had for peanuts. An amp built in the way prescribed would likely cost less than a single normal OT.

I suspect that you are been dumb deliberately because all of this has been set out clearly in this thread.

Shoog

piano3 said:
There is also an article with measurements on Steve Bench's site; if you go to Power amps, the link is a couple of paragraphs down. This is where I first saw this idea about 10 years ago.
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Me to.

Shoog
 
#160
Lingwendil said:
Running even Garter bias is worth a shot, my experience is very consistently good, the EL86 amplifier I put together is easily within .5mA, and in the past with the 6AS7 it was just as good. You have to match your resistors but that's pretty trivial. Nice and easy, same thing as individual cathode resistors, just doubled up. Only real drawback is on high bias tubes they waste a bit of voltage, but on stuff like the EL86, EL84, or even low-and-hot schemes they can be made to work. Even on the 6AS7 at a 100mA, 100V, -32 grid it isn't too bad, since you're already wasting some efficiency by going triode anyway.
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And then there's always CCSs instead of resistors 🙂 Automatic wear compensation, too.
 
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